Eddy current testing is a non-destructive test technique based on inducing electrical currents in the material being inspected and observing the interaction between these currents and the material. Eddy currents are generated by electromagnetic coils in the test probe, and monitored simultaneously by measuring probe electrical impedance. Since it is an electromagnetic induction process, direct electrical contact with the sample is not required; however, the sample material must be electrically conductive.
Various eddy current probes have been proposed for inspecting cylindrical or tubular components. Among many variations, the self-inductance type (the absolute and differential) and the transmit-receive type are in wide use. Many different coil configurations are also practised. Both ferromagnetic and non-ferromagnetic materials can be inspected. However, special care must be exercised for inspecting defects as will be discussed later.
In the past, bodies of ferromagnetic material have been inspected by a method such as the flux leakage method as taught, for example, in U.S. Pat. Nos. 3,091,733 (May 28, 1963, Fearon et al), 4,468,619 (Aug. 28, 1984, Reeves), and 4,602,212 (July 22, 1986, Hiroshima et al). In this method, the metal is magnetized in a direction parallel to its surface. At defects or where regions of the metal body are not uniform, some magnetic flux passes into the air and may be detected by sensors located nearby, thus giving an indication of the presence of faults, non-uniformity, etc.
U.S. Pat. No. 4,107,605 (Aug. 15, 1978, Hudgell) discloses an eddy current technique for detecting abnormalities in a pipeline of a ferromagnetic material. The eddy current probe includes a plurality of spiral sensing coils placed with their axes normal to the surface of the pipeline wall and connected on four legs of an AC bridge, thus compensating for lift-off. A biasing magnetic field by a permanent magnet permits distinguishing internal from external defects in weakly ferromagnetic tubes by comparing outputs from systems with and without biasing field.
U.S. Pat. Nos. 3,952,315 (Apr. 20, 1976, Cecco) and 2,964,699 (Dec. 6, 1960, Perriam) describe eddy current probes for use of testing weakly ferromagnetic tubes. They both include magnetic saturation means.
In U.S. Pat. Nos. 2,992,390 (July 11, 1961, de Witte) and 3,940,689 (Feb. 24, 1976, Johnson, Jr.) special electromagnetic ways of generating magnetic fields are taught in connection with the eddy current testing in that de Witte uses uniquely designed cores for transmit-receive coils using multiple test frequencies and Johnson, Jr. employs a solenoid wound about a core of a substantial length.
U.S. Pat. No. 3,437,810 (Apr. 8, 1969, Wood et al) describes a tube inspection apparatus having several different inspection instruments, one of which is an eddy current probe using two test frequencies.
None of these patents, however, is concerned with benefits obtained in operating simultaneously a plurality of eddy current probes at more than one operating point of the impedance diagram.